1. Field of the Invention
The present invention relates to an electronic flash device and, more particularly, it relates to a modified control system of the electronic flash device used for photo shooting.
2. Description of the prior art
A circuit to control on and off for discharge tube current by a semiconductor switching device connected in series to the flash discharge tube, in an electronic flash device used for an auxiliary light source for photo shooting, has been formerly realized by various kinds of semiconductor devices. As momentary current and voltage are great, formerly a thyristor inverter has been mainly used, but recently, a self-arc-extinguishing-type switching device has become high in performance, so that various kinds of such device's applications have been proposed.
FIG. 1 shows an example of the control circuit realized by using an insulated-gate bipolar transistor (IGBT). Similar invention is disclosed in U.S. Pat. No. 4,839,686. A high voltage power supply circuit 200 which generates a high voltage power of around 300 V by stepping cell voltage up by a DC-DC converter, is connected in parallel with a main condenser 101. The main condenser 101, a flash discharge tube 102 and an IGBT 104 are connected in series, and a trigger circuit 300 for giving the trigger signals to the flash discharge tube 102 is connected in parallel with the flash discharge tube 102.
Besides, a gate control circuit 400 which controls a operation of the IGBT 104 by turning on/off gate voltage of the IGBT 104, is connected to the gate of the IGBT 104, and the power supply of the gate control circuit 400 is obtained from a condenser 105 which gives DC voltage of around 30 V after receiving and smoothing rectified voltage from a diode connected to a tap of a voltage stepping-up coil of the DC-DC converter included in the high voltage power supply circuit 200.
In addition, a flash stop signal generating circuit 106 which generates a flash stop signal (a STOP signal) at predetermined time after a start of the flashing of the flash discharge tube 102, is connected to the flash discharge tube 102. The flash stop signal generating circuit 106 is connected to a reset terminal R of a R-S flip-flop 107. A flash start signal (a START signal) sent from a microcomputer and the like in a camera not shown, is given to a set terminal S of the R-S flip-flop 107. An output terminal Q of the R-S flip-flop 107 is connected to a base of a first step transistor within the gate control circuit 400.
High voltage of around 300 V and medium voltage of around 30 V are generated by working the DC-DC converter within the high voltage circuit 200 to charge the main condenser 101 and the condenser 105, respectively. High voltage from the high voltage power supply circuit 200 charges a trigger condenser 301 within the trigger circuit 300 through a resistor 303.
The gate control circuit 400 has to generate output voltage low enough for the IGBT 104 to be kept off till the flash start signal (the START signal) is given. Around total voltage of the high voltage from the high voltage power supply circuit 200, is applied to a collector of the IGBT 104.
When the flash start signal (the START signal) from a microcomputer and the like in a camera not shown, is put to the set terminal S of the R-S flip-flop 107 and then a "H" level from the output terminal Q of the R-S flip-flop 107 is put to the base of the first step transistor within the gate control circuit 400, the gate control circuit 400 gives voltage of around 30 V charged in the condenser 105 to the gate of the IGBT 104 to turn on the IGBT 104. In this way, the IGBT 104 is turned on between main electrodes, so that electric charges charged in the trigger condenser 301 within the trigger circuit 300 flows through a primary coil of a trigger transformer 302 to generate high voltage in the secondary coil. Thus the flash discharge tube 102 is triggered. When the flash discharge tube 102 is turned on, main discharge current flows in order from the main condenser 101, the flash discharge tube 102, the IGBT 104 to the main condenser 101, and the flash discharge tube 102 starts to flash.
Before the discharge from the main condenser 101 is over, the flash stop signal generating circuit 106 composed of a photo-diode, etc. detects photo signals from the flash discharge tube 102 and generates the flash stop signal (the STOP signal). The STOP signal is put to the reset terminal R of the R-S flip-flop 107, and a "L" level from the output terminal Q of the R-S flip-flop 107 is put to the first step transistor within the gate control circuit 400. In response to this, the gate control circuit 400 immediately cut off the output voltage, and quickly lowers the gate voltage given to the IGBT 104 till then down to the low voltage enough. In this way, the IGBT 104 turns off. As current having flowed through the flash discharge tube 102 till then is cut off, the flashing stops.
Collector voltage of the IGBT 104 rises and the trigger condenser 301 in the trigger circuit 300 is recharged to the original state, so that a state, before the flash start signal (the START signal) is given, is recovered. Thus, one cycle is over.
FIG. 2 shows a timing chart showing working of the circuit in the FIG. 1. Usually, as the flash start signal (the START signal) is generated by the microcomputer in the camera, the pulse width of the signal is comparatively great as shown in FIG. 2. On the other hand, there is a case where flash pulse width are needed to be short as some 10 micro-seconds. In this case, the flash start signal (the START signal) sometimes continues to be generated even after the flash stop signal generating circuit 106 outputs the flash stop signal (the STOP signal). In order to meet with it, a gate circuit has to be provided in the gate control circuit 400 in addition to the flip-flop circuit for generating the continued pulse width.
Though many of electronic circuits for controlling flashing duration of an electrode flash tube, are made integrated, as a circuit to perform the gate control of the IGBT 104 needs somewhat higher voltage, combination of discrete elements is needed to realize the circuit.
Thus, in former electronic flash devices, the IGBT 104 is controlled to be always off in waiting state till the flash discharge tube 102 starts to flash, so that high voltage is continuously applied between main electrodes of the IGBT 104. Besides it is necessary to extremely lessen leaking current from the IGBT 104 so as to enough charge the trigger condenser 301 in the trigger circuit 300. Consequently, high breakdown voltage of the IGBT 104 has to be secured, leading to increase of the chip size of the IGBT 104, so that lowering the cost was difficult.
In addition, in the gate control circuit 400, switching speed of the gate voltage is needed to be enough high upon on or off of the IGBT 104. In other words, as it is so composed that the flash discharge tube 102 is triggered through discharge of the capacity of the trigger condenser 301 by turn-on of the IGBT 104, the turn-on speed of the IGBT 104 is needed to be enough high for securing sufficient output voltage from the trigger circuit 300. In addition, the gate voltage is needed to be cut off enough quickly to prevent excessive flashing due to lags of the turn-off and the fall time of the IGBT 104.
Even in the IGBT device in which the gate current does not flow continuously, comparatively large chip size is needed for the IGBT 104 to control large current more than 100 A needed for the electronic flash device. Consequently, as parasitic capacity of the IGBT 104 becomes considerably high, around some 100 mA of output current from the gate control circuit 400 has to be flowed to drive the gate of the IGBT 104 at enough high speed. Accordingly, a comparatively large chip size is required for an output step transistor in the gate control circuit 400 to make it possible to flow around 500 mA at 50 V, and high breakdown voltage, around 50 V, is also required for the transistor which acts in conjunction with levels of the START and the STOP signals. Besides, the flip-flop circuit 107 for generating the continuous output voltage during the flash duration has to be provided, so that large number of parts are needed, resulting in becoming large in size of the circuit.
In addition, as charging and discharging to and from the trigger condenser 301 of the trigger circuit 300 are made through the coil of the trigger transformer 302 by switching of the IGBT 104, resonant voltage created from the capacity of the trigger condenser 301 and inductance of a coil of the trigger transformer 302, is generated upon the switching of the IGBT 104. As there are cases where reverse voltage is applied to the IGBT 104 upon turn-on of the IGBT 104 or surge voltage is applied to it upon the turn-off of the IGBT 104, the IGBT 104 has to be designed to endure these stresses, which disadvantageously causes the increase of cost.